Top Java Interview Questions and Answers for Freshers and Experience

Top Java Interview Questions and Answers for Freshers and Experience. In this article you can learn java interview questions for freshers and experienced. 

1.What exactly is Java?

Java is a platform-independent, high-performance, multithreaded, and portable high-level, object-oriented, resilient, and secure programming language. It was created in June 1991 by James Gosling. Because it comes with its own JRE and API, it’s also known as the platform.

2.What makes Java a platform-agnostic language?

Because the compiler compiles the code and then converts it to platform-independent byte code that can be run on many platforms, the Java language was designed to be independent of any hardware or software. The sole need for running the byte code is that the machine be equipped with a Java runtime environment (JRE).

3.Why isn't Java a pure object-oriented programming language?

Java is not a pure object-oriented language because it supports primitive data types such as byte, boolean, char, short, int, float, long, and double.

4.In C/C++, pointers are employed. Why is it that Java doesn't use pointers?

Beginner programmers should avoid using pointers because they are fairly difficult. The use of pointers can make Java difficult because it focuses on code simplicity. The use of a pointer might also lead to mistakes. Furthermore, when pointers are utilized, security is undermined since pointers allow people to directly access memory. By not including pointers in Java, a certain amount of abstraction is provided. Furthermore, the use of pointers might make garbage collection time-consuming and inaccurate. References are used in Java because, unlike pointers, they cannot be changed.

5.What exactly is the difference between an instance variable and a local variable?

Instance variables are variables that are available to all of the class’s methods. They are declared both outside and inside the methods. These variables describe an object’s attributes and are inextricably linked to it. All of the class’s objects will have their own copy of the variables to use. If any changes are made to these variables, just that instance will be affected, while all other class instances would stay unchanged.

6.In Java, explain public static void main(String args[]).

The entry point for any Java application is main(). It’s always written as public static void main in the source code (String[] args). public: The access modifier public is used to limit who has access to a method. Any Class will be able to access this Method because it is public. static: It is a keyword in Java that indicates that it is a class-based language. In Java, main() is made static so that it can be accessed without having to create a Class instance. Because main() is called by the JVM before any objects are created, and only static methods can be directly invoked via the class, the compiler will report an error if main is not made static.

7.Make a list of the characteristics of the Java programming language.

The Java Programming Language has the following features. Java is a simple language to learn. Java’s syntax is based on C++, making it easier to build programs in it.

Object-Oriented: Java adheres to the object-oriented paradigm, allowing us to maintain our code as a collection of various types of objects that include both data and behaviour. Read-once,

write-anywhere: Java supports the read-once, write-anywhere technique. On any machine, we can run the Java software. Java programmes (.java) are translated to bytecode (.class) that can execute on any machine.

Platform Independence: Java is a cross-platform programming language. It differs from other programming languages, such as C and C++, which require a platform to run. Java comes with its own platform, which is used to run its code. The execution of Java is not dependent on the operating system.

Java is safe because it does not make use of explicit references. Java also has the ByteCode and Exception handling concepts, making it more secure.

Java is a robust programming language because it makes extensive use of memory management. It is more robust thanks to features like automatic garbage collection, exception handling, and so on.

Java is architecturally neutral in the sense that it is not reliant on the architecture. The size of data types in C can change depending on the architecture (32 bit or 64 bit), however this is not the case with Java.

Java employs the Just-in-time (JIT) interpreter in conjunction with the compiler to execute programs.

High Performance: Because Java bytecode is “near” to native code, it is faster than other traditional interpreted programming languages. It’s still a tad slower than compiled languages.

Multithreaded: By defining numerous threads, we can construct Java applications that handle multiple tasks at once. The fundamental benefit of multi-threading is that each thread does not take up memory. It has a shared memory space. Threads are essential for multi-media, Web applications, and other applications.

Java is distributed because it makes it easy for users to construct distributed Java applications. For developing distributed applications, RMI and EJB are employed. This Java capability allows us to access files by calling methods from any system connected to the internet.

Java is a dynamic programming language. It allows classes to be loaded dynamically. It means that classes are loaded only when they are needed.

8.What exactly do you mean by Java virtual machine?

A virtual machine that allows a computer to run a Java program is known as a Java Virtual Machine. The JVM functions as a run-time engine that calls the main method in Java programming. The Java Virtual Machine (JVM) is a specification that must be implemented in a computer system. JVM compiles Java code into Bytecode, which is machine agnostic and similar to native code.

9.Why is Java platform agnostic?

Because its byte codes can operate on any system, regardless of the underlying operating system, Java is said to as platform independent.

10.Why isn't Java completely object-oriented?

Java isn’t entirely object-oriented since it uses eight primitive data types that aren’t objects: boolean, byte, char, int, float, double, long, and short.

11.In Java, what are wrapper classes?

Wrapper classes are used to convert Java primitives to reference types (objects). There is a class for each primitive data type. Wrapper classes are named thus because they “wrap” the primitive data type into a class object. Take a look at the graphic below, which shows several primitive types, wrapper classes, and function Object() arguments.

12.In Java, what are constructors?

A function Object() is a block of code in Java that is used to initialise an object. Its name must be the same as the class’s. It also doesn’t have a return type and is called automatically when an object is created.

13.What exactly do you mean by Java virtual machine?

A virtual machine that allows a computer to run a Java program is known as a Java Virtual Machine. The JVM functions as a run-time engine that calls the main method in Java programming. The Java Virtual Machine (JVM) is a specification that must be implemented in a computer system. JVM compiles Java code into Bytecode, which is machine agnostic and similar to native code.

Default Constructor: A default function Object() in Java is one that takes no arguments. In other words, default constructors are constructors that take no arguments and are produced by default if no alternative function Object() is specified by the user. Its primary function is to set the default settings for instance variables. It’s also widely used for object creation.

Parameterized Constructor: In Java, a parameterized function Object()  is a function Object() that can initialize instance variables with values specified by the user. In other words, parameterized constructors are constructors that accept arguments.

14.What is a singleton class in Java, and how do we create one?

A singleton class is one in which only one instance can be produced in a single JVM at any given moment. The function Object() of a class can be made private to make it a singleton.

15.What is the difference between Java Development Kit (JDK), Java Runtime Environment (JRE), and Java Virtual Machine (JVM)?

JVM stands for Java Virtual Machine, and it is an abstract machine that offers a runtime environment for Java bytecode execution. It is a specification that describes how the Java Virtual Machine works. Oracle and other companies have helped with its adoption. JRE is the name of the program that implements it. There are JVMs for a variety of hardware and software systems (so JVM is platform dependent). It is a runtime instance that is created when the Java class is executed. The JVM is divided into three parts: specification, implementation, and instance. 

JRE (Java Runtime Environment) is an acronym for Java Runtime Environment. It is the JVM implementation. The Java Runtime Environment (JRE) is a collection of software tools for creating Java applications. It’s responsible for creating the runtime environment. It is the JVM implementation. It is a real thing. It comprises a collection of libraries as well as other files that JVM uses during execution. JDK The term JDK stands for Java Development Kit. It’s a software development environment for creating Java applets and applications. It is a real thing. It includes JRE as well as developer tools. JDK is an implementation of any of Oracle Corporation’s Java Platforms, as listed below.

16.In Java, what is a package? Make a list of the numerous benefits of packages.

In Java, a package is a collection of similar classes and interfaces that have been grouped together. Developers may simply modularize code and optimise reuse by using packages. In addition, the code contained in the packages can be imported and utilised by other classes. I’ve highlighted a few of its perks below.

17.JVM allocates how many different sorts of memory areas?

The Class Area maintains per-class structures like the runtime constant pool, fields, method data, and method code. Heap: This is the runtime data location where the objects’ memory is allocated.
Stack: Frames are stored in Java Stack. It manages local variables and partial results, as well as invoking and returning methods. Each thread has its own JVM stack, which is built concurrently with the thread. Each time a method is called, a new frame is created. When a frame’s method invocation is finished, it is destroyed. The address of the Java virtual machine instruction presently being executed is stored in the PC register. The Native Method Stack contains all of the application’s native methods.

18.What exactly is a JIT compiler?

Compiler for Just-In-Time data: It is employed in order to enhance performance. JIT compiles bits of the bytecode with similar functionality at the same time, reducing the amount of time it takes to compile. The term “compiler” refers to a translator from a Java virtual machine’s instruction set to the instruction set of a specific CPU.

19.What is the name of the platform?

The hardware or software environment in which a piece of software is run is referred to as a platform. Software-based and hardware-based platforms are the two sorts of platforms. The software-based platform is provided by Java.

20.What are the primary distinctions between the Java and other platforms?

Other platforms may be hardware platforms or software-based platforms, but Java is a software-based platform.
Other hardware platforms can only contain the hardware components, whereas Java is processed on top of them.

21.Why aren't pointers used in Java?

Pointers are not used in Java since they are unsafe and add to the program’s complexity. Because Java is recognized for its code simplicity, introducing the concept of pointers will be incongruent. Furthermore, because the JVM is responsible for implicit memory allocation, pointers are discouraged in Java to avoid direct memory access by the user.

22.What are the fundamental concepts of Java OOPs?

Inheritance is the process by which one class inherits the properties of another.
Encapsulation: In Java, encapsulation is a way for encapsulating data and code into a single unit.
Abstraction: Abstraction is a technique for hiding implementation details from consumers and simply presenting them with functionality.
Polymorphism refers to a variable’s, function’s, or object’s ability to take numerous forms.

23.In Java, what is an infinite loop? Give an example to illustrate your point.

When a functional exit isn’t reached, an infinite loop is a Java instruction sequence that loops indefinitely. This type of loop might occur as a consequence of a programming error or as a result of the application’s behaviour. When the application is exited, an infinite loop will immediately end.

24.In Java, what is a classloader?

The Java ClassLoader is a component of the JVM (Java Virtual Machine) that handles class file loading. Whenever a Java application is run, the classloader loads it first. There are three built-in classloaders in Java.

Bootstrap ClassLoader

Extension ClassLoader

System/Application ClassLoader